An Ultra-Dense Network (UDN) is one of key technologies for 5th Generation (5G) mobile communications. A great number of Transmission Points (TPs) are deployed in indoor crowded scenarios such as an office building, a supermarket, a railway station, a gymnasium and a dense residential area, to improve coverage and increase the transmission rate of data services, so that a demand of 1000-times data service volume in future 5G mobile communications can be met. It is necessary to solve the synchronization/discovery problem between communication nodes (TPs or terminals) in the UDN.
There are three inter-TP synchronization methods in the related art: the first one is satellite navigation synchronization; the second one is synchronization via an ideal cable return by utilizing an Institute of Electrical and Electronics Engineers (IEEE) 1588V2 standard; and the third one is synchronization by monitoring a synchronization signal in an air interface between TPs (called air interface synchronization for short). The UDN is probably deployed in a seriously-shielded area, such as an indoor area, with a weak satellite signal or no satellite signal, and the satellite navigation synchronization cannot be realized. Moreover, due to the limitation of deployment cost, it is difficult to install ideal cable return for all TPs of the UDN. Therefore, an inter-TP air interface synchronization manner is mainly considered.
An inter-TP air interface synchronization method in the related art usually achieves air interface synchronization signal monitoring in a mute manner including a subframe-level mute manner and a special subframe Guard Period (GP)-level mute manner. The mute manner means that when a source TP transmits a synchronization signal, a target small base station stops transmitting own data in order to receive an air interface synchronization signal. Meanwhile, when a TP achieve multi-hop synchronization in a time division manner, a lower small station of the target small base station also keeps mute correspondingly, thereby avoiding interference. As shown in FIG. 1, at an nth (n is a positive integer greater than or equal to 1) subframe time, a TP1 transmits a synchronization signal, and a TP2 receives the synchronization signal mutely, and a TP3 also needs to be muted and does not transmit data to avoid interference to the TP2. Due to mute, neither the TP2 nor the TP3 can transmit data to a terminal at the nth subframe time, thereby increasing system overheads. Therefore, implementation of inter-TP air interface synchronization in the mute manner greatly reduces the using efficiency of radio resources.
A Long Term Evolution (LTE) Release 12 system solves the problems of terminal-based rapid TP discovery and synchronization through a discovery reference signal. The discovery reference signal includes a Common Reference Signal (CRS), a Primary Synchronization Signal (PSS)/Secondary Synchronization Signal (SSS), and a Channel State Information Reference Signal (CSI-RS), and overheads of the discovery reference signal are large, and interference between discovery reference signals and between a discovery reference signal and data will be caused.
An LTE system adds a Cyclic Prefix (CP) in front of each Orthogonal Frequency Division Multiplexing (OFDM) symbol, for solving the problems of interference between OFDM symbols and between subcarriers caused by multi-path delay and timing error. As the CP is longer, a supported maximum multi-path delay spread is longer, and corresponding coverage is larger. However, on the other hand, as the CP is longer, system overheads are larger. According to requirements for macro coverage, the LTE system supports two CP lengths namely a conventional CP of which the length is 144 Ts or 160 Ts, and an extended CP of which the length is 512 Ts, where Ts is an time unit of the LTE time domain, and Ts=1/30.72 μS. However, in the UDN, the coverage range of a communication node is greatly reduced (for example, the coverage range is tens of meters usually), and therefore there is great waste for the length of a traditional LTE CP.